1. Field of the Invention
This invention relates to linear light-emitting diode (LED) lamps and more particularly to a linear LED lamp with two shock protection switches, one at each of two ends of the lamp.
2. Description of the Related Art
Solid-state lighting from semiconductor light-emitting diodes (LEDs) has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (no hazardous materials used), higher efficiency, smaller size, and much longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. Meanwhile, as LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. In this trend, the potential safety concerns such as risk of electric shock, overheating, and fire become especially important and need to be well addressed.
LEDs have a long operating life of 50,000 hours. This is equivalent to 17 years of service period, assuming operating eight hours per day, every day. However, several factors may affect the operating life of an LED-based lamp. High operating temperature is most detrimental to both LEDs and the LED driver that powers the LEDs. While LEDs can operate 50,000 hours under a condition of good thermal management such as when using an efficient heat sink design, the lamp will not emit light when LED driver is broken, which happens if high-temperature air accumulates around the LED driver, and any of its electronic components fails. In spite of longevity of LEDs, the LED-based linear lighting system can operate only around 25,000 hours. Some issues related to system reliability during service life of an LED-based lighting system need also to be discussed.
In retrofit application of a linear LED tube (LLT) lamp to replace an existing fluorescent tube, one must remove the starter or ballast because the LLT lamp does not need a high voltage to ionize the gases inside the gas-filled fluorescent tube before sustaining continuous lighting. LLT lamps operating at AC mains, such as 110, 220, and 277 VAC, have one construction issue related to product safety and needed to be resolved prior to wide field deployment. This kind of LLT lamps always fails a safety test, which measures through lamp leakage current. Because the line and the neutral of the AC main apply to both opposite ends of the tube when connected, the measurement of current leakage from one end to the other consistently results in a substantial current flow, which may present risk of shock during re-lamping. Due to this potential shock risk to the person who replaces LLT lamps in an existing fluorescent tube fixture, Underwriters Laboratories (UL), use its standard, UL 935, Risk of Shock During Relamping (Through Lamp), to do the current leakage test and to determine if LLT lamps under test meet the consumer safety requirement.
Appliances such as toasters and other appliances with exposed heating filaments present this kind of hazard. When the line and the neutral wire reverse, the heating filaments can remain live even though the power switches to “off”. Another example is screw-in incandescent bulbs. With the line and the neutral wire reversed, the screw-in thread of the socket remains energized. These happen when the line and the neutral wires in the wiring behind the walls or in the hookup of sockets are somehow interchanged even with polarized sockets and plugs that design for safety. The reason why a consumer can widely use the appliances with heating filaments and screw-in light lamps without worrying about shock hazards is that they have some kinds of protections. The said appliances have protection grids to prevent consumers from touching the heating filaments even when they are cool. The screw-in light lamp receptacle has its two electrical contacts, the line and the neutral in proximity, recessed in the luminaire. When one screws an incandescent bulb in the receptacle, little shock risk exists.
As mentioned, without protection, shock hazard will occur for an LLT lamp, which is at least 2 feet long; it is very difficult for a person to insert the two opposite bi-pins at the two ends of the LLT lamp into the two opposite sockets at two sides of the fixture at the same time. Because protecting consumers from possible electric shock during re-lamping is a high priority for LLT lamp manufacturers, they need to provide a basic protection design strictly meeting the minimum leakage current requirement and to prevent any possible electric shock that users may encounter in actual usage, no matter how they instruct a consumer to install an LLT lamp in their installation instructions.
An easy solution to reducing the risk of shock is to connect electrically only one of two bi-pins at the two ends of an LLT lamp to AC mains, leaving the other dummy bi-pin at the other end of the LLT lamp insulated. In such a way, the line and the neutral of the AC main go into the LLT lamp through the bi-pin, one for the line and the other for the neutral. The electrically insulated dummy bi-pin at the other end only serves as lamp holder to support LLT lamp mechanically in the fixture. In this case, however, the retrofit of the existing fixture to enable LLT lamp becomes complicated and needs much longer time to complete, even for electrical professionals. The rewiring and installation costs will be too high for LLT lamp providers to replace conventional fluorescent tubes economically.
Referring to FIG. 1 and FIG. 2, a conventional LLT lamp 100 without protection switches comprises a plastic housing 110 with a length much greater than its radius of 30 to 32 mm, two end caps 120 and 130 each with a bi-pin on two opposite ends of the plastic housing 110, LED arrays 140 and 141 mounted on two PCBs 150 and 151, electrically connected in series using a connector 145, and an LED driver 160 used to generate a proper DC voltage and provide a proper current from the AC main and to supply to the LED arrays 140 and 141 such that the LEDs 170 and 171 on the two PCBs 150 and 151 can emit light. In some conventional LLT lamps, DIP (dual in-line package) rather than SMD (surface mount device) LEDs are used as lighting sources. Although SMD LEDs and the supporting PCB allow more efficient manufacturing, higher yield, higher lumen output and efficacy, and longer life than their DIP counterparts do, some LLT lamp providers still produce such DIP-based products. The two PCBs 150 and 151 are glued on a surface of the lamp using an adhesive with its normal parallel to the illumination direction. The bi-pins 180 and 190 on the two end caps 120 and 130 connect electrically to an AC main, either 110 V, 220 V, or 277 VAC through two electrical sockets located lengthways in an existing fluorescent tube fixture. The two sockets in the fixture connect electrically to the line and the neutral wire of the AC main, respectively. In some conventional LLT lamps, the LED driver wrapped by an insulation paper is inserted into the LLT lamp without being mechanically secured. Another drawback for this rough manufacturing process is poor heat dispersion, which may cause overheating over a certain period under high ambient-temperature operation and shorten the LED driver's life and the lamp's life as a whole due to poor air convection and heat accumulation inside the LLT lamp 100. In another conventional model, the circuitry of the LED driver 160 mixes with the LED arrays 140 on the PCB 150. Based on this configuration, there are two LED drivers: driver-1 160 and driver-2 161 as shown in FIG. 2. The drawback for this is that no sufficient number of LEDs is on the LED PCB, thus affecting lumen output and efficacy of the lamp. Another conventional type of LLT lamps uses two or more LED PCBs connected electrically in series. By using hard wires, the connections may not be reliable enough. Furthermore, the LED PCBs in some conventional LLT lamps are glued on the platform using adhesives, which may present another reliability issue because the PCB may peel off from the platform under adverse operating environments such as high temperature and high humidity. This is critical when the LED lamp is expected to service for 17 years.
To replace a fluorescent tube with an LLT lamp 100, one inserts the bi-pin 180 at one end of the LLT lamp 100 into one of the two electrical sockets in the fixture and then inserts the other bi-pin 190 at the other end of the LLT lamp 100 into the other electrical socket in the fixture. When the line power of the AC main applies to the bi-pin 180 through a socket, and the other bi-pin 190 at the other end is not in the socket, the LLT lamp 100 and the LED driver 160 are deactivated because no current flows through the LED driver 160 to the neutral. However, the internal electronic circuitry is still live. At this time, if the person who replaces the LLT lamp 100 touches the exposed bi-pin 190, which is energized, he or she will get electric shock because the current flows to earth through his or her body—a shock hazard.
Almost all LLT lamps currently available on the market are without any protection for such electric shock. The probability of getting shock is 50%, depending whether the person who replaces the lamp inserts the bi-pin first to the line of the AC main or not. If he or she inserts the bi-pin 180 or 190 first to the neutral of the AC main, then the LLT lamp 100 is deactivated while the internal circuitry is not live—no shock hazard.
An LLT lamp supplier may want to use only one shock protection switch at one end of an LLT lamp in an attempt to reduce the risk of shock during re-lamping. However, the one-switch approach cannot eliminate the possibility of shock risk. As long as shock risk exists, the consumer product safety remains the most important issue.